It is of course built in to OS X, just enable the IAC bus in your Audio / MIDI Setup application (Finder / Utilities). Under Mavericks 10.9, click on Window menu in the Audio / MIDI Setup application and click on 'show Midi Window'. Double-click on the IAC icon, activate the IAC driver ('the device is connected') and go to the sequencer menu of Jniz. Choose the 'IAC output' or 'Bus 1' and then take the IAC input into your DAW (see here).

With GarageBand for instance, there is nothing to param except the IAC driver (or bus 1) in Jniz sequencer menu. Choose a Steinway grand piano in the GarageBand you will get immediately a nice sound under Jniz.